4.3 issues dead #5 cylinder

this is the 2nd 4.3 ive gotten wiht a dead  #5  cylinder  both motors had/have high mileage  but seem to run good and have suffecient power none the less.     the last motor i tore down  and found NOTHING wrong with it. valves were good and functioning con rod was intact   piston was in good shape  everything was fine. since before the tear down i had replaced the rotor cap plugs and wires   i knew that wasnt the problem either.

well ive got another 4.3 dead on #5    changed the plugs and plug wires  cap  all that good stuff    still dead.    im not going to tear this one down because i suspect  nothing is wrong inside the motor because of the last time. 

sdoes anyone know why these motors do this?

Could be a bad fuel injector, bad cam lobe, or a vaccum leak or a bad O2 sensor